轧后冷却工艺对Si-Mn-Cr-Ni-Mo系超高强钢组织性能的影响

Effect of cooling process on microstructure and properties of Si-Mn-Cr-Ni-Mo ultra high strength steel after rolling

采用热轧后间断淬火+回火(IDQ+T)和在线配分(DQP)两种不同的工艺对Si-Mn-Cr-Ni-Mo系中碳低合金钢进行处理,利用SEM、XRD、EBSD研究冷却工艺对微观组织和力学性能的影响。结果表明:两种工艺下均得到板条马氏体和残留奥氏体的组织。经过轧后间断淬火+回火(IDQ+T),随淬火终冷温度升高,马氏体板条粗化,碳化物尺寸增加,残留奥氏体含量增加,强度降低,伸长率升高,韧性先升高后降低。组织中的粗大马氏体板条和尺寸较大的碳化物会降低韧性。在线配分(DQP)工艺得到的残留奥氏体含量最高,分布也更均匀,因此其伸长率和冲击功均明显增加,但残留奥氏体量增加同时会降低钢的强度,对屈服强度的影响最明显,导致屈强比降低。DQP处理实验钢的综合力学更优,抗拉强度超过1500 MPa、屈服强度超过1000 MPa,伸长率大于16%,-20℃冲击功达到26.8 J。

Two different processes of interrupt direct quenching ＋ tempering （IDQ ＋ T） and direct quenching and partitioning （DQP） were applied to a medium carbon low alloy steel Si-Mn-Cr-Ni-Mo, and microstructure and mechanical properties of the steel treated by different processes were investigated by means of SEM, XRD and EBSD. The results show that lath martensite and retained austenite are obtained by the two processes. After treating by IDQ ＋ T, the martensite lath broadening, the retained austenite volume fraction of the steel increase and the carbide can be coarsened with increasing of final cooling temperature, so the strength decreases, elongation increases and the impact energy increases at first and then decreases. The coalesced martensite and large size carbides are detrimental to the impact toughness. After the steel treating by DQP, the maximum of retained austenite content can be obtained and the distribution of retained austenite is more homogeneous. The elongation and impact energy obviously increases but strength decreases with retained austenite increasing, especially for the yield strength, so the yield ratio of the steel decreases. The steel treated by DQP shows better comprehensive mechanical properties, the tensile strength and yield strength are more than 1500 and 1000 MPa, respectively, the elongation is above 16% , and impact energy of 26.80 J at -20 ℃.